feat(discord): add mention_only option for @-mention responses (#518)

* feat(discord): add mention_only option for @-mention responses

Add MentionOnly config option to Discord channel. When enabled, the bot
only responds when explicitly @-mentioned, useful for shared servers.

- Add MentionOnly bool field to DiscordConfig
- Store botUserID on startup for mention checking
- Check m.Mentions before processing messages when MentionOnly is true
- Update config example and README documentation

* fix(discord): resolve race condition and strip mention from content

- Get botUserID before opening session to avoid race condition
- Add stripBotMention to remove @mention from message content
- Handles both <@USER_ID> and <@!USER_ID> mention formats

* fix(discord): skip mention_only check for DMs

DMs should always be responded to regardless of mention_only setting.
Added check to skip the mention_only logic when GuildID is empty.

* Update README.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Hua Audio <161028864+Huaaudio@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

config/config.example.json

@@ -57,7 +57,8 @@
     "discord": {
       "enabled": false,
       "token": "YOUR_DISCORD_BOT_TOKEN",
-      "allow_from": []
+      "allow_from": [],
+      "mention_only": false
     },
     "qq": {
       "enabled": false,